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# If 4 points are indicated on a line and 5 points are

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Intern
Joined: 18 Mar 2012
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If 4 points are indicated on a line and 5 points are [#permalink]

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16 May 2012, 06:52
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Difficulty:

15% (low)

Question Stats:

75% (01:59) correct 25% (02:17) wrong based on 282 sessions

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If 4 points are indicated on a line and 5 points are indicated on another line that is parallel to the first line, how many triangles can be formed whose vertices are among the 9 points?

A. 20
B. 30
C. 40
D. 70
E. 90

Tried doing a few options, but couldn't get it right and the answer provided left me more confused!
[Reveal] Spoiler: OA
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Re: If 4 points are indicated on a line and 5 points are [#permalink]

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16 May 2012, 07:58
15
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Expert's post
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alexpavlos wrote:
If 4 points are indicated on a line and 5 points are indicated on another line that is parallel to the first line, how many triangles can be formed whose vertices are among the 9 points?

A. 20
B. 30
C. 40
D. 70
E. 90

Tried doing a few options, but couldn't get it right and the answer provided left me more confused!

Approach #1:
There are two types of triangles possible:
With two vertices on the line with 4 points and the third vertex on the line with 5 points --> $$C^2_4*C^1_5=30$$;
With two vertices on the line with 5 points and the third vertex on the line with 4 points --> $$C^2_5*C^1_4=40$$;

Total: $$30+40=70$$.

Approach #2:

All different 3 points out of total 4+5=9 points will create a triangle EXCEPT those 3 points which are collinear.
$$C^3_{9}-(C^3_4+C^3_5)=84-(4+10)=70$$ (where $$C^3_4$$ and $$C^3_4$$ are # of different 3 collinear points possible from the line with 4 points and the line with 5 points, respectively).

Hope it's clear.
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Re: If 4 points are indicated on a line and 5 points are indicat [#permalink]

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16 May 2012, 07:04
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say line A with 4 points and line B with 5 points. and triangle is PQR.
If P is on line A and QR on line B we can have 4C1*5C2=40 triangles.
If P is on line B and QR on line A we can have 4C2*5C1=30 triangles.
Total =70 triangles.
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Re: If 4 points are indicated on a line and 5 points are [#permalink]

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29 Dec 2012, 01:27
2
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Not that fun this question I wonder how often I would see such a question on test day.

Solution:

* * * * * (line B)

* * * * (line A)

Choose 2 points from line A and a point from line B: = 4!/2!2! * 5!/1!4! = 30
Choose 1 point from line A and 2 points from line B: = 4!/1!3! * 5!/2!3! = 4 * 10 = 40

Combine possiblities: 30 + 40 = 70
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Re: If 4 points are indicated on a line and 5 points are [#permalink]

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06 Aug 2015, 15:29
1
KUDOS
Expert's post
dukenukem wrote:
Bunuel wrote:
alexpavlos wrote:
If 4 points are indicated on a line and 5 points are indicated on another line that is parallel to the first line, how many triangles can be formed whose vertices are among the 9 points?

A. 20
B. 30
C. 40
D. 70
E. 90

Tried doing a few options, but couldn't get it right and the answer provided left me more confused!

Approach #1:
There are two types of triangles possible:
With two vertices on the line with 4 points and the third vertex on the line with 5 points --> $$C^2_4*C^1_5=30$$;
With two vertices on the line with 5 points and the third vertex on the line with 4 points --> $$C^2_5*C^1_4=40$$;

Total: $$30+40=70$$.

Approach #2:

All different 3 points out of total 4+5=9 points will create a triangle EXCEPT those 3 points which are collinear.
$$C^3_{9}-(C^3_4+C^3_5)=84-(4+10)=70$$ (where $$C^3_4$$ and $$C^3_4$$ are # of different 3 collinear points possible from the line with 4 points and the line with 5 points, respectively).

Hope it's clear.

Thanks Bunuel. That was great. Do you by any chance have other problems similar to this that you know of?

Combination problems are located at search.php?search_id=tag&tag_id=52
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Re: If 4 points are indicated on a line and 5 points are [#permalink]

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19 Jul 2014, 21:45
See Image below for what my scratch paper looked like on this one
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Gmat_56_resized.jpg [ 101.22 KiB | Viewed 5098 times ]

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Re: If 4 points are indicated on a line and 5 points are [#permalink]

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19 Jul 2014, 22:47
Bunuel wrote:
alexpavlos wrote:
If 4 points are indicated on a line and 5 points are indicated on another line that is parallel to the first line, how many triangles can be formed whose vertices are among the 9 points?

A. 20
B. 30
C. 40
D. 70
E. 90

Tried doing a few options, but couldn't get it right and the answer provided left me more confused!

Approach #1:
There are two types of triangles possible:
With two vertices on the line with 4 points and the third vertex on the line with 5 points --> $$C^2_4*C^1_5=30$$;
With two vertices on the line with 5 points and the third vertex on the line with 4 points --> $$C^2_5*C^1_4=40$$;

Total: $$30+40=70$$.

Approach #2:

All different 3 points out of total 4+5=9 points will create a triangle EXCEPT those 3 points which are collinear.
$$C^3_{9}-(C^3_4+C^3_5)=84-(4+10)=70$$ (where $$C^3_4$$ and $$C^3_4$$ are # of different 3 collinear points possible from the line with 4 points and the line with 5 points, respectively).

Hope it's clear.

I really like your 2nd approach
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Re: If 4 points are indicated on a line and 5 points are [#permalink]

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31 Jul 2015, 01:02
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Re: If 4 points are indicated on a line and 5 points are [#permalink]

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06 Aug 2015, 14:54
Bunuel wrote:
alexpavlos wrote:
If 4 points are indicated on a line and 5 points are indicated on another line that is parallel to the first line, how many triangles can be formed whose vertices are among the 9 points?

A. 20
B. 30
C. 40
D. 70
E. 90

Tried doing a few options, but couldn't get it right and the answer provided left me more confused!

Approach #1:
There are two types of triangles possible:
With two vertices on the line with 4 points and the third vertex on the line with 5 points --> $$C^2_4*C^1_5=30$$;
With two vertices on the line with 5 points and the third vertex on the line with 4 points --> $$C^2_5*C^1_4=40$$;

Total: $$30+40=70$$.

Approach #2:

All different 3 points out of total 4+5=9 points will create a triangle EXCEPT those 3 points which are collinear.
$$C^3_{9}-(C^3_4+C^3_5)=84-(4+10)=70$$ (where $$C^3_4$$ and $$C^3_4$$ are # of different 3 collinear points possible from the line with 4 points and the line with 5 points, respectively).

Hope it's clear.

Thanks Bunuel. That was great. Do you by any chance have other problems similar to this that you know of?
Re: If 4 points are indicated on a line and 5 points are   [#permalink] 06 Aug 2015, 14:54
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